This invention relates to methods and apparatus for withdrawing effluent from a solids-contacting vessel and particularly to an improved method and apparatus for effluent withdrawal to obtain improved vessel performance.
In solids-contacting vessels, liquid having suspended or dissolved solids is introduced into the vessel for removal of a portion of the solids, particularly those originally in the liquid or those precipitated from the liquid having a density greater than the liquid in which they are carried. In such vessels, the untreated liquid is introduced into a lower portion of the vessel and removed from an upper portion of the vessel. During the liquid's upward travel, most of the suspended solids are removed, typically with the aid of precipitating and/or flocculating agents added to the vessel.
One such vessel configuration (described in detail in U.S. Pat. No. 4,146,471) provides flow characteristics that optimize solids contacting time and vessel efficiency. The vessel comprises an upper end and a lower end with a substantially conical portion having a large diameter at the upper end and a small diameter at the lower end, an inlet for an untreated liquid at the vessel lower end, and means for causing the untreated liquid, fed by the inlet to the vessel lower end, to flow in a helical path upwardly in the conical portion to a sludge-gathering zone in the large diameter conical portion, where the solids precipitate and/or agglomerate and separate in a revolving sludge blanket layer below but near the upper end of the vessel, with clarified liquid continuing above the sludge blanket. An outlet for clarified liquid is typically positioned at a fixed location somewhere near the liquid surface in the vessel.
This vessel shape provides optimum clarifying because the rotational and upward velocities of liquid in the tank decrease as the liquid nears the top, although the flow volume remains the same. As the liquid velocity decreases, the solids have an opportunity to settle and/or be conglomerated at the sludge blanket. The rotating sludge blanket is preferably gradually withdrawn through a central, and vertically adjustable, downcomer. Other means for withdrawing sludge may be used.
As is apparent from the above description, the relative velocity of the liquid throughout the vessel plays an important role in vessel performance. Under ideal circumstances, withdrawal of liquid at a fixed location along the surface will result in consistent effluent quality and previously known solids-contacting vessels are equipped with only fixed effluent outlets.
In practice, other factors affect the quality of the effluent withdrawn from the vessel, including: vessel shape; the vessel's interior smoothness; the location, size, and orientation of the liquid inlet means; the location, size, and orientation of any structures located within the vessel; the volume and density of the sludge blanket; the relative flowrate of the incoming liquid going to each of multiple liquid inlet means; start-up and shut-down procedures; varying liquid temperatures; varying liquid flow rates; and others. These factors influence effluent quality because they affect upward and rotational liquid velocities which can cause short-circuiting of the liquid through the vessel and result in shorter solids-contacting time and/or localized areas of high vertical liquid velocities that carry suspended solids that would normally settle up through the clarification portion of the vessel to the effluent withdrawal means. Further, zones of liquid along a horizontal radius of the vessel near the sludge blanket may have higher upward and/or rotational velocities than others. Higher upward and/or rotational velocities generally result in less effective solids removal and higher percentage of solids retained in effluent. Thus, controlling high velocity zones or removing effluent from zones of desired velocities results in improved vessel performance. Unfortunately, these zones cannot be accurately and precisely determined at design of the vessel and they vary due to changes in the flow rate, temperature chemical feed rates, etc. which can also vary over relatively short periods of operating time. Known fixed effluent withdrawal units cannot adapt to changing conditions within the vessel.